This invention relates to the calculation of a mass flow rate of material flowing through a Coriolis flowmeter. More particularly, this invention relates to compensating a measured flow rate for error in the flow rate caused by the density of the material being measured. Still more particularly, this invention relates to determining when the density of a material is causing an unacceptable error in the mass flow rate and compensating for the error caused by density in the flow rate.
It is known to use Coriolis effect mass flowmeters to measure mass flow and other information of materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and Re. 31,450 to J. E. Smith of Feb. 11, 1982. These flowmeters have one or more flow tubes of a curved configuration. Each flow tube configuration in a Coriolis mass flowmeter has a set of natural vibration modes, which may be of a simple bending, torsional, radial, or coupled type. Each flow tube is driven to oscillate at resonance in one of these natural modes. The natural vibration modes of the vibrating, material filled systems are defined in part by the combined mass of the flow tubes and the material within the flow tubes. Material flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter. The material is then directed through the flow tube or flow tubes and exits the flowmeter to a pipeline connected on the outlet side.
A driver applies a force to the flow tube. The force causes the flow tube to oscillate. When there is no material flowing through the flowmeter, all points along a flow tube oscillate with an identical phase. As a material begins to flow through the flow tube, Coriolis accelerations cause each point along the flow tube to have a different phase with respect to other points along the flow tube. The phase on the inlet side of the flow tube lags the driver, while the phase on the outlet side leads the driver. Sensors are placed at two different points on the flow tube to produce sinusoidal signals representative of the motion of the flow tube at the two points. A phase difference of the two signals received from the sensors is calculated in units of time.
The phase difference between the two sensor signals is proportional to the mass flow rate of the material flowing through the flow tube or flow tubes. The mass flow rate of the material is determined by multiplying the phase difference by a flow calibration factor. This flow calibration factor is determined by material properties and cross sectional properties of the flow tube.
It is a problem that properties of a material may effect mass flow rates measured by Coriolis flowmeters. Some properties of a material that may effect measured flow rates include density, temperature, pressure, and viscosity. In most cases, a Coriolis flowmeter is designed to be insensitive to the errors caused by these properties. In other cases, meter electronics compensate for errors in the measured mass flow rate caused by these properties. For example, meter electronics commonly compensate for errors caused by the temperature and pressure of a material.
Sometimes the error caused by the properties of a material are insignificant under normal operating conditions and the error in flow rate is not corrected. However, a property of a material may cause unacceptable errors after a certain threshold is surpassed. For example, the density of a material often does not affect the flow rate in most flow meters at most densities. However, in low flow rate Coriolis flowmeters, it has been observed that density of the material effects the measured flow rate of the material after a certain threshold. For purposes of this discussion, low flow rate is 5 lbs./ minute or less. At this time, it is unknown what causes these errors.
Therefore, it is desirable to determine when the density of the measured material surpasses a threshold and to compensate for the error in flow rate caused by the density.
The above and other problems are solved and an advance in the art is made by the provision of a method and apparatus for compensating for errors in measured mass flow rates caused by density in a Coriolis flowmeter. One advantage of this invention is that errors in a measured flow rate attributable to density are corrected. A second advantage of this invention is that the compensation is more accurate than other systems as non-linear equations that more precisely fit measured data are used to determine a compensation factor. A third advantage of this invention that the compensation may only occur after the density has surpassed a certain threshold where the error caused by density surpasses an unacceptable level. This reduces the amount of computation needed to provide an accurate flow rate.
A determination of a mass flow rate that is compensated for density in accordance with this invention is performed in the following manner. First a material flows through a vibrating conduit in a Coriolis flowmeter. The conduit is vibrated and sensors affixed to the conduit generate signals representing the motion of the conduit. Signals from sensors affixed to the vibrating conduit are received by the meter electronics. An uncompensated flow rate of the material is then calculated by the meter electronics from the received signals. A density compensation factor is then determined using uncompensated flow rate and non-linear information relating density to errors in flow rate. A density compensated flow rate is then generated by applying the density compensation factor to the uncompensated flow rate.
In accordance with this invention, the meter electronics may also calculate a density of the material from the signals received from the sensors. The calculated density then may be compared to a threshold to determine whether the density surpasses a threshold value. If the density does surpass the threshold value the density compensated flow rate is generated. Otherwise the uncompensated flow rate is output.
In an alternative embodiment, a linear density compensation factor may be calculated using the uncompensated flow rate and linear information relating errors in the mass flow rate to the density of the material if the threshold is not exceeded. The compensated flow rate can be calculated by applying said linear density compensation to said uncompensated mass flow rate.
In accordance with this invention the density compensation factor may be determined by inserting the uncompensated flow rate into a N order polynomial equation relating density to flow rate error data wherein N is greater than 1. The N order polynomial is a curve fit of density to error rate in measured mass flow. The polynomial may be generated by performing an N order curve fit of the density to flow rate error data wherein N is greater than 1.